Where are the Night Shift and Dark Mode buttons?

Open App Menu > Tools > System Colors. That menu groups Enable Dark Mode, Enable True Tone and Enable Night Shift. In the menu-content customization list, their visibility is managed together as System Colors.

If System Colors is missing, go to Settings > Menu > Customize App Menu Contents…. Under Tools, find System Colors and select Visible. When Expanded keeps it behind the menu’s expansion option, while Hidden removes it from the menu.

Return to the app menu and open System Colors to use its individual settings. Making the menu visible does not create True Tone or Night Shift support on an unsupported setup. HDR-related settings can also turn these color features off. The menu can show a notice when that happens.

See System Colors for the toggles and related adjustments, and menu contents for visibility settings.